Patent GB-1508641-A (GB)
Title: Hydrostatic transmission control system
IPC: F16H61/42
Filed: 1975-02-11
Abstract: 1508641 Fluid-pressure servomotors systems EATON CORP 11 Feb 1975 [28 Feb 1974 2 Oct 1974] 5780/75 Heading G3P [Also in Divisions F2 and H3] A control for a hydrostatic transmission having a variable displacement swash-plate pump 14 supplying pressure liquid to a fixed or variable displacement motor 30 comprises an electronic circuit, Fig. 3, having a feedback circuit 164 producing a first electric signal proportional to the actual pump displacement, a command circuit 160 producing a second electric signal proportional to a desired pump displacement, an amplifier 212 producing an output signal proportional to the difference between the first and second signals and electro-hydraulic means to change the displacement of the pump 14 at a first rate when the difference between the first and second signals exceed a pre-determined value and at a second rate less than the first rate when the difference between the first and second signals is less than the pre-determined value. The first signal is produced by a potential divider 200 having a movable contact 198 fast with the pump swash-plate 28 and the second signal is produced by a potential divider having a moving contact 188 fast with a manual control lever 18. An actuator circuit 170 initially energizes a solenoid 174 when the command signal (second signal) exceeds the actual signal (first signal) and, after a time delay determined by a circuit 186, energizes a solenoid 176 if the difference between the first and second signal exceeds the predetermined value. When the actual signal exceeds the command signal the actuator circuit initially energizes the solenoid 176 and after a time delay energizes the solenoid 174 if the difference between the signals exceeds the predetermined value. The solenoids 174, 176 actuate pilot valves 38, 40 controlling a main control valve 36 and the rate of change of pump displacement. The control valve 36 selectively connects a pump 24 to hydraulic motors 54, 60 which adjust pump displacement. The pump 24 additionally supplies pressure liquid through lines 124, 126 and restrictions 134, 136 to servo cylinders 120, 122 acting on opposite ends of the spool of valve 36. When the spool of valve 36 is in its central neutral position, as shown, the servos 120, 122 are connected through lines 128, 130 to the pilot valves 38, 40 and through restrictions 110, 84, to exhaust. When the command signal exceeds the actual signal energization of the solenoid 174 changes over the pilot valve 38 to vent the servo 120 whereupon the spool of the valve 36 moves left to connect the pump 24 through a line 50 to the motor 54 which increases pump displacement. In addition the valve connects a line 66 from the motor 60 to exhaust via the restriction 84 for a slow rate of change of pump displacement. If the command signal exceeds the actual signal by the predetermined value subsequent energization of the solenoid 176 changes over the pilot valve 40 to permit rapid flow from the motor 60 and thus a rapid change in pump displacement. When the actual signal approaches the command signal the solenoid 176 is de-energized and the pilot valve 40 returned by a spring 284 to the position shown in which exhaust from the motor 60 passes through the restriction 84 for slow change of pump displacement. When the actual signal is equal to the command signal the solenoid 174 is de-energized and the pilot valve 38 returned to the position shown to cause a pressure build-up in the servo 120 whereupon a spring 144 returns the spool of the valve 36 to its central neutral position. When the lever 18 is moved to reduce pump displacement the pilot valve 40 is initially actuated to cause rightward movement of the spool of valve 36 which thus connects the pump 24 through line 66 to the motor 60 which reduces pump displacement. If a large reduction in pump displacement is required the pilot valve 38 is additionally actuated to effect rapid change in pump displacement. When the pump displacement approaches the selected value the pilot valve 38 is returned to its initial position, as shown, to slow the rate of change of pump displacement. When the pump displacement attains the selected value the pilot valve 40 and control valve 36 revert to the positions shown. If the pump 24 fails a valve 151 inter-connects the lines 50, 66 to the pump displacement adjusting motors 54, 60 and springs 153, 155 in the motors 54, 60 move the pump swash-plate to its zero displacement position. The valve 151 is normally held in the position shown by the output pressure by the pump 24. In Fig. 4 (not shown) a variable displacement motor is provided and its displacement is controlled by an electronic control circuit Fig. 5 (not shown) and valve assembly similar to those provided for the pump.